The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for Government purposes without the payment of any royalties thereon or therefor.
The present invention relates to a plasma generator and accelerator and, more particularly to a low power, compact plasma accelerator that can be used for satellite propulsion, drag reduction and station-keeping, or for ion plasma material processing in a vacuum.
There is a need for a simple, low power, light-weight, compact, high specific impulse electric propulsion device to satisfy mission requirements for micro and nano-satellite class missions. Satisfying these requirements entails addressing the general problem of generating a sufficiently dense plasma within a relatively small volume and then accelerating it in a way that generates a net thrust reaction force in a desired linear direction. Known means for ion generation and propulsion generally require relatively large containment volumes in order to achieve reasonable ionization efficiencies, therefore new means are needed in order to achieve effective scaled-down propulsion devices.
Recent prior art electric propulsion devices and plasma accelerators are commonly some form of Hall effect thrusters (Hall accelerators or Hall engines). A conventional Hall effect thruster generally comprises an accelerating channel arranged along an axis with an anode and a propellant source at a first, generally closed, end of the channel, and a cathode (electron source) at a second, generally open, end of the channel. The cathode and anode establish an electric field with a gradient generally aligned with the axis of the channel. A system of magnets is arranged so that a magnetic field crosses the channel.
To continue the description of the Hall effect thruster, an exemplary thruster is presented comprising an annular accelerating channel extending circumferentially around the axis of the thruster and also extending in an axial direction from a closed end to an open end. The anode is usually located at the closed end of the channel, and the cathode is positioned outside the channel close to its open end. Means is provided for introducing a propellant, for example xenon gas, into the channel and this is often done through passages formed in the anode itself or close to the anode. A magnetic system applies a magnetic field in the radial direction across the channel and this causes electrons emitted from the cathode to move circumferentially around the channel. Some but not all of the electrons emitted from the cathode pass into the channel and are attracted down the electric field gradient towards the anode. The radial magnetic field deflects the electrons in a circumferential direction so that they move in a spiral trajectory, accumulating energy as they gradually drift towards the anode. In a region close to the anode the electrons, collide with atoms of the propellant, causing ionization. The resulting positively charged ions are accelerated by the electric field towards the open end of the channel, from which they are expelled at great velocity, thereby producing the desired thrust. Because the ions have a much greater mass than the electrons, they are not so readily influenced by the magnetic field and their direction of acceleration is therefore primarily axial rather than circumferential with respect to the channel. The ion stream is at least partially neutralized by those electrons from the cathode that do not pass into the channel.
Conventionally, the required radial magnetic field has been applied across the channel using an electromagnet having a yoke of magnetic material which defines poles on opposite sides of the channel, i.e. one radially inwardly with respect to the channel and the other radially outwardly with respect to the channel. An example is shown in European patent specification 0 463 408 which shows a magnetic yoke having a single cylindrical portion passing through the middle of the annular channel and carrying a single magnetizing coil; and a number of outer cylindrical members spaced around the outside of the accelerating channel and carrying their own outer coils. The inner and outer cylindrical members are bolted to a magnetic back plate so as to form a single magnetic yoke.
A recent example of the Hall effect thruster is disclosed in U.S. Pat. No. 5,847,493 (Yashnov, et al.; 1998) entitled xe2x80x9cHall Effect Plasma Acceleratorxe2x80x9d. The described invention in the U.S. Pat. No. 5,847,493 Patent comprises the use of magnets (permanent or preferably electric) wherein the magnetic poles are defined on bodies of material which are magnetically separate in order to allow greater freedom in selecting the dimensions of the thruster, particularly the length in the axial direction relative to the diameter of the accelerating channel.
U.S. Pat. No. 5,751,113 (Yashnov, et al.; 1998), discloses a closed electron drift Hall effect plasma accelerator with all magnetic sources located to the rear of the anode. It is stated that this makes it possible to provide a Hall effect accelerator with an optimum distribution of magnetic field inside the acceleration channel by means of a simpler and less heavy arrangement using a single source of magnetic field, such as a single coil or permanent magnet. As in all Hall effect thrusters, the magnetic field lines (13, as seen in FIG. 2) extend laterally across the accelerating channel (1) over the anode (2) and propellant gas source (3) located at the closed end of the channel (see FIG. 1).
A problem common to the Hall effect thrusters is one of scaling its size. In general, it is difficult to scale down Hall effect thrusters appreciably because of the magnetic field requirements. In smaller engines, the large transverse magnetic fields required can hamper ion flow, thereby reducing the ion beam current. This is particularly problematic for such engines generating milliamp magnitude beams for micro-thruster applications, wherein small thrust to power ratios make Hall effect thrusters impractical for micro-satellite applications. Another scaling problem is that electromagnets do not scale well with size reduction because of heating issues and coil size required to achieve the desired field.
Hall effect thrusters generally employ hollow cathodes, and preferably employ electromagnets, thereby requiring fairly complicated, and thus heavier, control systems in order to control electromagnet current, gas flow in both the anode and the discharge electrode, and cathode discharge current. Adding to the problems of complexity and weight, the hollow cathode consumes propellant.
U.S. Pat. No. 6,075,321 (Hruby; 2000), discloses a Hall field plasma accelerator with an inner and outer anode, designed to deal with problems of wall heating and sputtering that are characteristic problems with Hall effect thrusters.
A non-Hall effect thruster is described by U.S. Pat. No. 4,937,456 (Grim, et al.; 1990), that discloses a dielectric coated ion thruster comprising a cathode chamber (12) from which free electrons flow into an attached ionization chamber (14) along with a flow of ionizable gas atoms. According to the abstract and to column 6 of the detailed description, the free electrons are accelerated by a positive potential applied to the interior surface of the ionization chamber, causing the electrons to collide with atoms of the gas with sufficient kinetic energy to create ions. The positively charged ions are accelerated toward a negatively charged perforated grid plate (24, 112), pass through the grid plate, and exit in a focused beam, providing thrust in the opposite direction. A plurality of bar magnets (20, 22, 108, 110) are arranged in a spaced apart circular array around the cathode chamber with a pole face of each of the magnets tangentially aligned with wall sections (16, 18, 102, 104) of the ionization chamber. The bar magnets define an axial geodesic picket fence arrangement that extends circularly about the cathode chamber, wherein the pole faces of adjacent bar magnets that are in contact with the ionization chamber alternate north and south polarity, so that a magnetic field extends between the opposite pole faces of adjacent bar magnets. Although magnetic field lines are not illustrated, it can be seen from FIGS. 1 and 5, for example, that the magnetic field lines will arch from pole to pole to create a scalloped line around the circumference of the ionization chamber with cusps occurring at each pole. As stated in column 7 of the detailed description, as a negatively charged electron is accelerated toward the wall sections, the magnetic field interacts with the moving charge, causing the electron to experience a force directed generally at a right angle to its forward velocity. In response to this force, the electrons are caused to spiral in a helical path, thereby extending the mean path of the electrons to increase the probability that the electrons may strike an atom and ionize it. Since the magnetic field lines that confine the plasma within the ionization chamber bend laterally away from the magnet poles (forming cusps), the surfaces of the poles are not well protected by the magnetic field and would normally be exposed to erosion due to impacts by high-energy electrons or ions, therefore dielectric coating (42) is provided to protect them from sputtering. Likewise, the outer surface of an emitter tube (28, 61, 128), and the inner and outer surfaces of the grid plate, are coated with a dielectric material to protect them from sputtering erosion.
Problems inherent in conventional ion thrusters with grids (e.g., U.S. Pat. No. 4,937,456) include significant erosion issues for which dielectric coatings are needed to help provide protection, thereby adding weight and complexity. Furthermore, the use of grids along with charged chamber walls require the use of multiple power supplies, thereby complicating the power processor unit. Finally, gridded systems have inherently lower thrust density capability relative to gridless concepts.
It is known that plasma accelerators can be used for material processing in a vacuum by means of plasma ion interaction with materials. U.S. Pat. No. 6,380,684 (Li, et al.; 2002) discloses a plasma generating apparatus and semiconductor manufacturing method which generates a high-density plasma in a rectangular chamber using magnetron, high frequency discharge plasma generation, i.e., a high frequency oscillating electric field that interacts with magnetic fields to produce electrons and ions in a plasma. An annular-rectangular (xe2x80x9cfistulousxe2x80x9d) discharge electrode (14) is in close proximity to concentric annular-rectangular permanent magnets (15, 16) that are arranged axially on either side of the discharge electrode to generate magnetic field lines that loop over the discharge electrode to cusps that are on either axial side of the electrode. Rectangular parallel plate electrodes (17, 18) at the top and bottom of the chamber are either grounded or connected to a second high frequency source. The top electrode 17 is used, for example, as gas diffusion plate for diffusing a discharge gas or a process gas, wherein the top electrode (17) is a perforated gas shower plate (37).
It is an object of the present invention to provide a compact plasma accelerator that overcomes problems such as those described hereinabove for known devices, thereby providing sufficient thrust density to provide a simple, low power, light-weight, compact, high specific impulse electric propulsion device to satisfy mission requirements for micro and nano-satellite class missions.
According to the invention, a compact plasma accelerator has components including a cathode electron source, an anode, a source of ionized gas, and a magnetic field source, wherein: the components are held by an electrically insulating body having a central axis, a top axial end, and a bottom axial end The magnetic field source comprises: a cylindrical magnet having an axis of rotation that is the same as the axis of rotation of the insulating body, and magnetized with opposite poles at its two axial ends; and an annular magnet coaxially surrounding the cylindrical magnet, magnetized with opposite poles at its two axial ends such that a top axial end has a magnetic polarity that is opposite to the magnetic polarity of a top axial end of the cylindrical magnet. The source of ionized gas is a tubular plenum that has been curved into a substantially annular shape, positioned above the top axial end of the annular magnet such that the plenum is centered in a ring-shaped cusp of a magnetic field generated by the magnetic field source, and having one or more capillary-like orifices spaced around the top of the plenum such that an ionizing gas supplied through the plenum is sprayed through the one or more orifices. The plenum is electrically conductive and is positively charged relative to the cathode electron source such that the plenum functions as the anode; and the cathode electron source is positioned above and radially outward relative to the plenum.
According to the invention, the compact plasma accelerator is further characterized in that the cylindrical magnet and the annular magnet are preferably permanent magnets.
According to the invention, the compact plasma accelerator is further characterized in that the plenum is preferably enclosed in an electrically insulating material having an axially-oriented hole above each of the one or more orifices. Furthermore, the body preferably has a cavity opening upward and sized to enclose the plenum in combination with an electrically insulating cover plate that covers the cavity and the plenum, and the cover plate has the axially-oriented holes.
According to the invention, the compact plasma accelerator is preferably further characterized in that a field shaping plug is mounted in the insulating body above the cylindrical magnet such that the field shaping plug""s axis of rotation is the same as the axis of rotation of the cylindrical magnet, the field shaping plug is a cylinder that comes to a conical point at its top axial end, and is made of a ferromagnetic material; such that the field shaping plug concentrates the magnetic field emerging from the top axial end of the cylindrical magnet to form a very narrow pointed cusp above the field shaping plug. Furthermore, the field shaping plug is preferably made of mild steel.
According to the invention, the compact plasma accelerator is preferably further characterized in that the bottom axial end of the insulating body is covered by a backing plate made of a ferromagnetic material such that the backing plate concentrates the magnetic field at the bottom axial end of the cylindrical magnet and the annular magnet. Furthermore, the backing plate is preferably made of mild steel.
According to the invention, the compact plasma accelerator is further characterized in that the cathode electron source is preferably a hot filament, a field emitter type cathode or a very low flow rate hollow cathode type device. Furthermore, the hot filament cathode electron source preferably comprises one or more wires shaped in a ring that circumnavigates the plenum above and radially outward relative to the plenum. Also, preferably a single power source powers the hot filament cathode electron source and also the positively charged, anodic, plenum. It is within the terms of the invention to use a separate power source to power the filament supply.
According to the invention, the compact plasma accelerator is, further characterized in that the cathode electron source may be one or more hollow cathodes. According to the invention, the compact plasma accelerator is further characterized in that the electrically insulating body is preferably made using a ceramic material. Furthermore, the ceramic material is preferably a machinable ceramic.
According to the invention, a method of producing and accelerating an ion beam comprises the steps of:
a) providing a magnetic field with a cusp that opens in an outward direction along a centerline that passes through a vertex of the cusp;
b) providing an ionizing gas that sprays outward through at least one capillary-like orifice in a plenum that is positioned such that the orifice is on the centerline in the cusp, outward of the vertex of the cusp;
c) providing a cathode electron source, and positioning it outward of the orifice and off of the centerline; and
d) positively charging the plenum relative to the cathode electron source such that the plenum functions as an anode.
According to the invention, the method of producing ,and accelerating an ion beam preferably further comprises the steps of:
e) using a hot filament for the cathode electron source; and
f) powering both the hot filament cathode electron source and the positively charged, anodic, plenum with a one or more power sources
According to the invention, the method of producing and accelerating an ion beam preferably further comprises the steps of:
g) using a hot filament for the cathode electron source; and
h) powering both the hot filament cathode electron source and the positively charged, anodic, plenum with one or more power sources.
According to the invention, the method of producing and accelerating an ion beam preferably further comprises the step of using permanent magnets for providing the magnetic field.